Method for chemical vapor infiltration of refractory substances, especially carbon and silicon carbide

ABSTRACT

A method for isothermic, isobaric chemical vapor infiltration (CVI) of refractory substances, especially of carbon (C) and silicon carbide (SiC), based on diffusion in a porous structure, whereby the pressure of the gas or partial pressure of an educt gas contained in the gas and the dwell time of the gas in the reaction zone are set at a given temperature in the reaction zone so that a deposition reaction occurs in the porous structure in the area of pressure or partial pressure of the saturation adsorption of the gaseous compounds forming the solid phase, saturation adsorption meaning that the deposition speed remains substantially constant at increased pressure of the gas or partial pressure of the educt gas. The reaction of the educt gas is limited in such a way that no more than 50% of the elements in the educt gas as it flows through the reaction zone are deposited as a solid phase in the porous structure.

The invention concerns a method of chemical gas phase infiltration ofrefractory substances, in particular carbon and silicon carbide and theprefered method of application.

BACKGROUND

Methods of deposition of solid phases on substrates by decomposition ofvolatile or gaseous compounds which contain the solid phase elements aredesignated as chemical vapor deposition. If this deposition takes placein the open pores of a porous substrate or in the cavities of a porousstructure, then it is known as chemical vapor infiltration. Chemicalvapor deposition (CVD) and chemical vapor infiltration (CVI) are ofsignificance primarily with respect to the deposition and infiltrationof refractory materials such as carbon, carbides, nitrides, borides,oxides etc. (see for example W. J. Lackey, in Encyclopedia ofComposites, Vol. 1, edited by Stuart M. Lee, VCH Publishers, Inc., NewYork, 1990, pg. 319). CVI methods allow a densification of structure or,when the porous structure consists of fibers, an introduction of amatrix and, with this, the production of composite, strengthened fibermaterials.

Both chemical vapor deposition as well as chemical vapor infiltrationare extremely complex processes. In the chemical vapor deposition ofcompounds such as e.g. silicon carbide, there exists the additionalproblem that it is in general difficult to attain solid phase depositionin stoichiometric compositions for larger parts in the generation of acoat of even thickness.

In chemical gas phase infiltration there is another and particularproblem, that the volatile or gaseous starting compounds must betransported into the depth of the pores before dissolving. Ifdecomposition occurs on the surface of the porous structure of in thepore entrances, then the pores become clogged. The pores are then notfilled, which is the whole aim of the process.

Various embodiments of methods for chemical vapor infiltration (CVI) areknown.

Procedurally the simplest to perform are methods of isobaric andisothermic chemical vapor infiltration. In this method the entireprocess space exists at constant temperature and pressure. Here,however, only very low pressures or partial pressures of educt gases canbe used, when necessary with addition of inert or dilution gases, sothat extremely long infiltration times are required.

In order to shorten the infiltration times it is proposed according toWO 95/16803 that for the chemical vapor infiltration of silicon carbideusing methyltrichlorosilane (MTS) as educt gas, the educt gas should bepreheated to temperatures well above the decomposition temperature ofMTS i.e. to 960° C. to 1050° C. while at the same time setting pressuresup to 25 kPa and to remove silicate components from the gas phase at theoutlet of the reaction zone. Preheating the MTS to such hightemperatures leads to a high rate of deposition of the substances addedwith the gas, which in turn achieves a high production speed but at thesame time leads to uneven deposits, particularly on the surface, andtherefore to minimal extents of pore filling.

Optimal or maximal pore filling is therefore only possible at extremelyslow deposition or infiltration rates (e.g. W. V. Kotlensky, inChemistry and Physics of Carbon, Vol. 9, edited by P. L. Walker and P.A. Thrower, Marcel Dekker, New York, 1973, pg. 173).

In order to successfully realize infiltration, low pressures, andparticularly low partial pressures are recommended. The pressuresrealized under the conditions of industrially applied chemical vaporinfiltration lie at least one to two orders of magnitude below normalpressure. Starting compounds are partially mixed with inert gases sothat their partial pressure, and with it the deposition rate can befurther lowered. Due to the low partial pressures, extremely longinfiltration times of up to several weeks are required.

Since the isobaric and isothermic methods failed in achieving rapidproduction and high degrees of pore filling, the development of newmethods was attempted, known as pressure gradient, temperature gradientand pressure switching methods. Such methods are for example known fromNyan-Hwa Tai and Tsu-Wei Chou, Journal of American Ceramic Society 73,1489 (1990).

In the vacuum pressure pulsation method, the process pressure iscontinually varied to support the diffusion. The disadvantage of thismethod lies in the cost of the apparatus as well as in the filtrationtimes, which are still very long.

Another well-known method is the temperature gradient method (e.g. U.S.Pat. Nos. 5,411,763, 5,348,774). In this method heat is removed from theside of the porous substrate facing the process gas stream by suitablemeasures, for example by cooling by the stream. The side of the poroussubstrate opposite to the gas stream is adjacent to a heating element.It is in this way that a temperature gradient crucial to the method isestablished normal to the surface of the substrate. The surfacetemperature on the cold side is adjusted with the gas stream such thatno, or at least very little deposition takes place. It is in this waythat narrowing of the pores in this region is avoided. The disadvantageof this method is the very high gas throughput necessary for cooling.The low yield of deposited material entails long production times. Muchequipment is needed for the heating.

In a further known embodiment of CVI methods (DE 41 42 261) the gas isstreamed through the porous substrate on the basis of forced convectionwhereby a pressure gradient is established. The infiltration time can bekept relatively short. After a certain level of pore filling however,the streaming through of the porous structure becomes more difficult.

From U.S. Pat. No. 4,580,524, a CVI method is known whereby temperatureand pressure gradient techniques are combined with one another. In thisway relatively short production times can be achieved. The disadvantageof such a method is the complicated reactor construction.

The task which provided the basis for the invention was to create a CVImethod by which a high level of pore filling during a pre-set productiontime could be achieved, or alternatively, a shorter production timeachieved for pre-set pore filling levels.

SUMMARY OF THE INVENTION

The task is solved by means of the invention of a method incorporatingthe features according to claim 1.

There is no question about it that an isobar, isotherm (under “isotherm”we understand a temperature constancy as great as possible in thereactor and even if this is not absolutely achieved, the term isothermis still used) chemical gas phase infiltration is the simplest toexecute and at the same time the most universal technical method.

The inventive method for chemical vapor infiltration of refractivematerials, especially of carbon (C) and of silicon carbide (SiC), isbased on diffusion in the porous structure and functions isothermally,in other words no temperature gradients are intentionally established.The infiltration takes place under isobaric conditions, e.g. the porousstructure to be infiltrated is streamed with a gas in a reaction zone,but is not subjected to a flow through such that an appreciable pressuregradient is formed. Here, the gas pressure or the partial pressure of aneduct gas contained within the gas and the persistence of the gas in thereaction zone are adjusted for the prescribed temperature in thereaction zone such that in the porous structure a deposition reaction inthe pressure and partial pressure region of the saturation adsorption ofthe gaseous and volatile compounds forming the solid phase exists.Saturation adsorption means that the deposition rate remainssubstantially unchanged or is negligibly increased by a raising of thegas pressure or of the partial pressure of the educt gas. This meansthat the deposition or infiltration rate of the solid phase, in otherwords the refractory substances, formally progresses as a reaction ofzeroth order, but any case as a reaction of order significantly lessthan one. Furthermore, the gas pressure or the partial pressure of aneduct gas contained within the gas and the persistence of the gas in thereaction zone are adjusted for the prescribed temperature so that thetransformation of the educt gas is limited such that in the flow throughof the reaction zone no more than 50%, and preferably 10% to 25% of thesolid-forming elements introduced into the educt gas are deposited inthe porous structure. In addition, the porous structure is subjected toflow-through gas linearly from the bottom to the top through aperturesof substantially identical width from 1 to 50 mm, preferably less than25 mm.

This means for example that compared to conventional methods,substantially higher pressures and partial pressures of the educt gasare set, which are higher than those of known isobaric, isothermalmethods, in order to enable high or the highest possible depositionrates. In light of the state of the art, these pressures are at least sohigh as to be characterized as disadvantageous.

Let it therefore be emphasized once again that such conditions allow notonly maximum infiltration rates to be achieved but also maximum extentsof pore filling.

In order to simultaneously achieve good pore filling while applying highpressure at the same time, the method according to the inventionrequires very special reaction control. Here, the choice of startingcompound is of special importance. This means that for the deposition ofany kind of refractory substance, special starting compounds must bechosen which best meet the requirements of the inventive method.

In order to explain the nature of the method according to the invention,it is necessary to describe the chemistry and kinetics of thedecomposition of the starting compounds during simultaneous depositionof the solid phase. For reasons of simplicity, this is done by way ofexample of the deposition of the element carbon. Equation (1) shows adiagram with methane as the starting material.

In this diagram C₁ is methane, C₂ and C₄ stand for C₂ and C₄hydrocarbons such as e.g. ethylene, acetylene and butadiene. C₆ meansbenzene and if appropriate, benzene substitutes. The above reactionsequence describes the gas phase reactions in pyrolysis of methane asknown for example in the petrochemical industry (crack reactions). Theformation of higher aromatic compounds such as naphthalene, anthracene,pyrene etc. in the gas phase should be avoided as far as possible asthey lead to nucleation in the gas phase and therefore to the undesiredsoot formation. The arrows in equation (1) pointing downwards areintended to illustrate the formation of carbon, in other words, itsdeposition on the substrate. The missing arrow under methane (C₁) meansthat this or the C₁ radicals arising from it are not in a position toform carbon or at least only if the speed can be reduced. The increasingthickness of the arrows from C₂ to C₆ is meant to make clear that thedeposition speed increases from C₂ to C₆ at a considerable speed.

The diagram shows therefore, that methane is ideal as a startingcompound of carbon for a chemical gas phase infiltration. Due to itssmall molar mass in comparison with other hydrocarbons, methane candiffuse in the pores with maximum speed but carbon deposition only comesabout after the C₂, C₄ hydrocarbons and benzene are formed.

The use of methane for chemical vapor infiltration of carbon is not new;on the contrary, methane or natural gas alone find preferedimplementation in the chemical vapor infiltration processes of carbondue to their inexpensiveness. The example however, clearly shows theimportance of the starting compound for chemical gas phase infiltration.Analogous for example, would also be silicon carbide for chemical gasphase infiltration. In this case in technological areas i.e. inindustrial processes, methylchlorosilane and in particularmethyltrichlorosilane CH₃SlCl₃ are used almost exclusively. Especiallyfor this starting compound it is stated that it already contains theelements of the solid phase to be deposited, namely silicon and carbonin stoichiometric composition. Indeed, in the deposition of siliconcarbide from methyltrichlorosilane there normally occurs co-depositionof free carbon. In order to avoid this, mixtures ofmethyltrichlorosilane and hydrogen are used. If one regards thedecomposition reactions, methyltrichlorosilane seems less well suited tovapor infiltration (equation 2):

CH₃SiCl₃→CH₃+SiCl₃   (2)

Radicals immediately occur which can form silicon carbide by methylradical reaction with a silicon atom and by silyl radical reaction witha carbon atom on the surface of the growing silicon carbide layer. If onthe other hand the thermally considerably more stable compounds SiCl₄and CH₄ are used, these can diffuse primarily into the pores beforeradicals are formed, from which then silicon carbide arises. In realitythe gas phase chemistry is more complex, but consideration of theseelementary and primary reactions is sufficient to illustrate theprinciple.

According to the invention the concentrations and the partial pressuresof the starting compounds of the process in the gas volume adjacent tothe pore openings, which corresponds to the free reactor volume, are setto equally high levels as at the entrance to the reactor. Decompositionof the starting compounds in the free reactor volume is largely avoidedby suitable setting of the deposition temperature and most importantlythe persistence time. For this reason only extremely short persistencetimes are realized. By this, decomposition reactions within the freereactor volume which are due to extended persistence times and whichlead to undesirable deposition on the surface or in the pore entrancesof porous substrates are avoided.

The decomposition reactions can be especially impressively documentedwith the aid of equation (1) for methane. Over longer periods in thefree reactor volume, C₂ and C₄ species and benzene can form, whichautomatically leads to carbon deposition on the surface and at the poreopenings. At high partial pressures of the starting materials thisproblem is so critical that conventional methods with high partialpressures lead to extremely poor results.

According to the invention, advantage is taken of the fact that shortpersistence times in the free reactor volume do not mean short times inthe pores. Despite the short persistence time set according to theinvention, the decomposition reactions can accordingly occur in thepores, and the solid phase can thus be deposited. The duration of thepersistence time is set such that a low percentage transformation of theeduct gas, i.e. a low deposition rate in solid phase of the elementscarried in the educt gas which form the solid phase in the porousstructure is attained. The concrete persistence time to set in order toattain the desired deposition rate for a given process temperature andpressure can be determined by test experiments by fixing the compositionand amount of the educt gas used as well as by weighing the structurebefore and after vapor infiltration.

The short persistence times which are set are a necessary but notsufficient feature of the method according to the invention. Shortpersistence times imply high flow rates, in which there is a strongtendency to form eddy currents. Due to the circulation flow in eddycurrents, the persistence time is locally increased, leading to morepronounced deposition, in chemical vapor infiltration by necessity onthe surface of the substrate. At high pressures and partial pressures aswell as by the method according to the invention this is especiallyproblematic due to the high concentrations and partial pressures of thestarting materials. However, by subjecting the porous structure tolinear flow from bottom to top, the circulation flow caused byconvection are dampened. According to the invention the flow is appliedthrough apertures of substantially identical width of 1 to 50 mm,preferably less than 25 mm. These aperture widths are just big enough toavoid having to establish a deleterious substantial pressure gradient inorder to attain the flow rate as prescribed by the persistence time, yetat the same time are small enough so that the formation of circulatoryflow and the resulting local increase in persistence time can beavoided.

The combination according to the invention of in themselvescounterproductive measures, namely the setting of high pressures on theone hand and of low deposition rates on the other, has the effect thatchemical vapor deposition according to the method of the inventionattains a combination of high production speed and a high extent of porefilling which up to now has not been anywhere near possible. A furtherincrease in the extent of pore filling is attained by applying gasthrough small apertures, whereby the required production time is reducedas well by the avoidance of deposits on the surface of the part whichmight have to be filed off.

Reactor constructions and reactor parts are used in performing themethod which allow implementation of the special conditions of themethod in a technically and financially optimum manner.

The reaction temperature plays a critical role in the method of theinvention. It influences both the rate of reaction i.e. thedecomposition and deposition reactions as well as the diffusion into orwithin the pores, although in very different ways. For the reaction rater equation (3) is valid in the simplest case, under the assumption of afirst order reaction.

|r|=k*c _(i)   (3)

For the diffusion speed j equation (4)

|j|=D*c _(i)   (4)

In these equations:

k=rate constant of the reaction

D=diffusion coefficient

c_(i)=concentration of a component

The reaction rate constant k and the diffusion coefficient D areinfluenced by the temperature as follows: (equation (5) and (6)):

k˜exp(−E_(A)/RT)   (5)

D˜T^(1.5)   (6)

In these equations:

E_(A)=activation energy

R=universal gas constant

T=temperature in Kelvin

For carbon deposition the activation energies are in partextraordinarily high, in the case of methane, the activation energy is105 kcal/mol. A small change in temperature therefore causes a verylarge change in the reaction rate (equation (3) and (5)), whereas theinfluence on diffusion is relatively small (equation (4) and (6)). Atlower temperatures the diffusion is greatly favoured in comparison tothe reaction, which is of advantage to the in-pore deposition, which isthe deposition inside the pores. This conclusion is given by the stateof state of the art. However, at the same time a slow reaction rate isthe outcome, which leads to long production times.

According to the invention, however, a hitherto unnoticed aspect isexploited. The equations (3) and (4) show that the reaction rate and thediffusion rate are dependent on the concentration c_(i) or on thecorresponding partial pressure p_(i). If these are increased, thereaction rate and the diffusion rate are accelerated equally, whereby atleast no disadvantage arises for the diffusion rate with respect to thereaction rate. The influence of a decreased temperature on the reactionrate can be compensated by a significant increase of the concentrationc_(i) or the partial pressure p_(i).

In chemical vapor deposition and therefore also in chemical vaporinfiltration, the reaction in the decisive step is not a homogeneousreaction but a heterogeneous one, so that the surface reaction has to beconsidered in the deposition of the carbon. In the case of aone-center-reaction, the order of the reaction speed changes withincreasing partial pressure formally from one to zero; the latter isknown as a saturation adsorption. This signifies for equation (3) thatthe reaction rate becomes independent of the concentration c_(i) or thepartial pressure p_(i) (equation (7)):

|r|=k   (7)

If in this case the concentration or the partial pressure is increased,the reaction rate remains constant, the diffusion rate howeverincreases, which is of excellent advantage for an in-pore deposition.Low temperatures and high concentrations or partial pressures aretherefore simultaneously excellent criteria for the method of theinvention.

According to the invention it is possible that the gas flow to which theporous structure is subjected contains a significant portion of inert ordilution gas, e.g. nitrogen, argon etc. According to a preferedembodiment of the invention, however, no inert or dilution gas is addedto the gas. This means that gas such as certain natural gases which bynature contains a small amount of inert or dilution gas can certainly beused. However, no additional inert or natural gas should be added tolower the partial pressure of the starting materials.

The omitting of inert gas according to the prefered embodiment of theinvention has multiple advantages. In comparison to the process in thestate of the art, high concentrations or partial pressures of thestarting materials are achieved. A higher concentration of the startingmaterials in the free reactor volume increases the concentrationgradient towards the pores and hereby also the motive force of thediffusion into the pores. As high concentrations or partial pressures aspossible are also of essential importance for another reason. If thepartial pressures are too small, the starting materials completely reactas early as at the entrances to the pores and no deposition occurs inthe depth of the pores. As a result the pores would become cloggedwithout being filled.

Furthermore, inert gas molecules can collide with the starting materialsin the gas phase, accelerating the decomposition of the startingmaterials. Inert gas is therefore not absolutely inert. In the case ofcarbon deposition or carbon infiltration with methane as startingmaterial, for example, carbon deposition on the outer surface or at theentrance to the pores of the porous substrate is decreased by omittinginert gas so that higher degrees of pore filling are made possible.

The exclusion of addition of inert or dilution gas in the preferedembodiment of the method of the invention is carried out bearing this inmind, despite the fact that this exclusion increases the partialpressure and thereby has an assumedly worsening effect on the filling ofthe pores.

If the partial pressure is increased with the requirement that inert anddilution gas is not used, this entails an increase in the totalpressure, which in turn has a negative influence on the diffusioncoefficient D and thus also on the diffusion rate j as shown in equation(8):

D˜p⁻¹   (8)

p=total pressure

This influence of the total pressure on the reaction rate does notrepresent any problem for the method according to the invention. This isfor two reasons:

(1) in depositions with pure gas without inert gas the saturationadsorption is achieved at a lower pressure, since the partial pressureis identical to the total pressure.

(2) In the case of saturation absorption, the ratio of reaction rate anddiffusion rate is not influenced by increasing pressure of the startinggases, as both the reaction rate according to equation (7) and thediffusion rate according to equation (9) are independent of the totalpressure:

|j|=D _((p)) *c˜1/p*p˜1   (9)

D_((p)) corresponds to equation (8)

In taking advantage of these effects the invention achieves with highpressures of the starting gases the creation of optimum conditions foran in-pore deposition. In the case of multi-center-reactions, thekinetics of the surface reaction are substantially more complex.Nevertheless it is also true in this case that the maximum reaction ratein saturation adsorption of all species, which at all events must bestriven for, at a constant partial pressure ratio of the startingmaterials does not lead to any further increase in the reaction rate ofthe surface reaction. The same considerations as for theone-center-reaction are therefore valid.

Decisive for the method according to the invention are therefore theright choice of starting materials, high pressures and especially highpartial pressures of the starting materials and low temperatures, whichin conjunction with a short persistence time, allow only minordecomposition of the starting materials in the free reaction volume.

According to the invention, suitable low temperatures can be realizedfor vapor infiltration with the respective starting materials.

According to a prefered embodiment, temperatures in the range of 1.000to 1.200° C. are set in the reaction zone for vapor infiltration of C.

According to a further prefered embodiment, temperatures in the range of900 to 1.100° C. are set in the reaction zone for the vapor infiltrationof SiC.

According to a prefered embodiment of the invention, gas is passed overthe porous structure which comprises a by-product generated during vaporinfiltration. Preferably hydrogen gas is added to the educt gas for thevapor infiltration of C. The volume ratio of methane or natural gas toadded hydrogen is 20:1 to 2:1, preferably 10:1 to 5:1. Hydrogen chloridegas is preferably added to the gas educt for vapor infiltration on SiC.The molar ratio of methyltrichlorosilane to added hydrogen chloride gasis preferably 5:1 to 1:5, more preferably 3:1 to 1:2.

Two further advantageous effects are achieved by the addition of H₂ tothe educt gas methane:

(1) The diffusion coefficient of methane is substantially increased,meaning that pore diffusion is favored in a decisive manner.

(2) The volume increase caused by the complete decomposition of methaneinto carbon and hydrogen in the pores of the porous structure isdecreased by the hydrogen content, and the rinsing effect is accordinglyweakened.

The addition of hydrogen has the effect that even at low partialpressures of methane it is possible to regulate the conditions of thesaturation adsorption. However, according to the invention, it is alsopossible to achieve saturation absorption even for lower partialpressures of methane independent of the amount of hydrogen present bydecreasing the temperature. This measure is therefore also valid if nohydrogen is added to the methane. Even a relatively small temperaturedecrease on the order of magnitude of 20° C., for example from 1,100° C.to 1,080° C., is sufficient to significantly decrease the partialpressure of methane at which saturation adsorption is achieved. Suchsmall decreases in temperature barely influence the diffusion into thepores of the porous structure since the diffusion coefficients exhibitonly minimal temperature dependence. The effect of a temperaturedecrease of 20° C. on the lowering of the partial pressure of methane atwhich saturation adsorption is achieved is comparable to the effect of ahydrogen addition at a methane/hydrogen ratio of 6:1.

According to the invention it is therefore possible to adjust thesaturation adsorption as a decisive criterion of the method by means oftwo different and largely independent parameters within a relativelybroad range of pressures or partial pressures. With this it is possibleto adjust the conditions of the-infiltration according to the invention,especially in respect to the saturation adsorption for each porousstructure, independent of its porosity, distribution of pore radii orpore structure, and especially independent of its geometric dimensions.It is especially advantageous that upon variation of the decisiveparameters temperature and partial pressure of hydrogen, no unwanteddisadvantages for the method are created. To achieve an infiltrationwhich is as fast as possible, high pressures or partial pressures areprefered, especially pressures of 15 to 25 kPa.

Analogous controlling means for the saturation adsorption also exist inthe case of a chemical vapor infiltration of silicon carbide. A decreasein temperature always leads to a saturation adsorption at low partialpressures. It is therefore possible to adjust the pressure or thepartial pressure its optimum value, e.g. also to increase it, byincreasing the temperature. At the point of saturation adsorption thismeans that the deposition rate does not increase further, yet thediffusion stream in the pores does increase since the motive force ofthe diffusion is stronger due to the higher partial pressure. The roleplayed by hydrogen with respect to the saturation adsorption during theinfiltration of C is assumed by hydrogen chloride during theinfiltration of silicon carbide. In using hydrogen chloride thesaturation adsorption can be achieved at lower pressures or partialpressures as well; the side effect on diffusion is not at as importantas in the case for hydrogen. The problem of volume increase during thetotal decomposition of methyltrichlorosilane into silicon carbide andhydrogen chloride in the pores of the porous structure is a priori evenlarger than during the total decomposition of methane; it is howeveralready diminished drastically by the use of mixtures for the depositionor infiltration of silicon carbide consisting of methyltrichlorosilaneand hydrogen in which the amount of hydrogen surpasses the proportion ofmethyltrichlorosilane manyfold.

The method according to the invention is preferably conducted in areactor having a specific reactor construction, or at least in a reactorwith a special mounting, which are also the subject matter of theinvention. Since brake disks made of carbon fiber-reinforced carbon areamong the most important products which are manufactured by chemicalvapor infiltration in large quantity, specifically of carbon into carbonfiber structures, and since they furthermore have potential for growth,the principle of reactor construction or of the reactor components willbe explained by the following example.

In order to gain a better overview, the carrying or holding devices inthe vertical cross section have been omitted. The fitted parts can becomposed of ceramic material; preferably however carbon of graphite isused. The thickness of the fitted parts and the recesses correspond tothe thickness and diameter of the brake discs. The starting materialstreams through the interstices, whose width is not represented toscale, between the fitted parts. Where there is a correspondingly smalldistance between the fitted parts, a very high speed can be achievedwith a relatively small volume of flow and low tendency to formvortices. This is particularly important, since only a minimum turnoverof the starting materials can be permitted due to the decomposition.

The design of the reactor according to FIG. 1 can always be used whenrelatively flat parts or, as in the present example, when parts withlarge diameters or length to thickness ratio are to be infiltrated. Ifthe task concerns the infiltration of parts with medium or smalldiameter/length to thickness ratio, then a reactor construction inaccordance with FIG. 2 is appropriate for the method. It shows ahorizontal cross section of the reactor. In this case the reactorconstruction is composed of a full material, preferably of carbon orgraphite, with vertical pipes containing the parts which are to beinfiltrated. The same construction can also be used when the radialinfiltration of flat parts is favoured instead of axial infiltration,for example in layers of fibre weaves.

Taking the example of this reactor construction, the principle ofinfiltration of small or the smallest partial pressures, as used intechnology, can be explained so as to be easily understood. As aconsequence of the very low pressure, the diffusion rate is high, sothat the diffusion into the pores is favored. This does not exclude thatthe starting compounds already react at the opening to the pore, namelyas a result of adsorption to active centres. At the beginning ofinfiltration deposition occurs only in the lower area of the reactorsince the starting materials are completely converted i.e. used up. Withcontinued infiltration and filling of the pores and with it thereduction of the internal surface, the consumption of the startingmaterials in this area declines so that the infiltration now takes placein the middle area. As a whole, this means that the reaction frontmigrates through the reactor from the bottom to the top. As well as thelow infiltration rate in itself, it also leads to an increase ininfiltration time.

In principle, the method according to the invention can be carried outwith the reactors described. Since there is a low level of turnover ofthe starting materials however, a circulation system could be ofeconomic advantage. In order to put this into practice it is necessaryto consider the gas leaving the reactor, primarily in terms of itscomposition. As an example, carbon deposition from methane is to beobserved. In accordance with equation (1) high aromatic compounds canoccur as reaction products C₂- to C₄-hydrocarbons and benzene, and if soin traces. These would cause considerable disturbance because of theirhigh tendency towards the formation of carbon, even after supplementingthe gas with fresh methane. It is therefore necessary to remove them asfar as possible. The most detrimental compounds, benzene and higharomatic compounds can be removed in principle by means of condensing.One variation which is to be prefered and in which the C₂ to C₄hydrocarbons are also removed, uses their considerably higherdecomposition speed during the formation of carbon, in comparison withmethane, as in equation (1). The pre-condition for the removal of thecarbon from the gas flow is a large surface for its deposition. Suchconditions can be produced in a fluidised bed with fine vortex material.This can be a carbon powder or a ceramic powder. During operation of thefluidised bed a certain percentage of fresh particles are addedcontinually, an appropriate percentage is then taken off with the aid ofan overflow in the fluidised bed. This makes stationary operationpossible. If ceramic powders such as aluminium oxide powder are used,the carbon coated particles can be used for energy production by burningoff the carbon and then re-introduced into the fluidised bed.

For selective removal of the undesired hydrocarbons with at the sametime minimising of the methane decomposition, the fluidising bed must beoperated below the temperature of the deposition or infiltrationreactor. In the case of carbon infiltration with methane, a typicaltemperature is 1050 to 1100° C. The fluidising bed is operated at alower temperature of about 100 K. This procedure for the separation ofthe undesired hydrocarbons is possible for two reasons. The first reasonis described by equation (1). The second, even more important reason isthe substantially higher activating energy of the methane decompositionin comparison to the carbon formation from the C2 to C4 hydrocarbons,benzene and higher aromatic compounds. A slight drop in temperaturetherefore makes it possible for a working off reaction of the undesiredhydrocarbons, which are still supported by the large surface in thefluidising bed because the high reaction speed is only lowered slightlywith the sinking temperature. In the case of methane on the other hand,the decomposition reaction is practically frozen to a stop by this dropin temperature, i.e. it does not work or works only to a very limitedextent.

In the course of previous observations up to now, no account was takenof the fact that hydrogen is created both in the decomposition ofmethane in the deposition or infiltration reactor and also in the carbondeposition in the fluidising bed. If this was not removed, there wouldbe a continual addition of hydrogen in circulation system gas. It istherefore necessary, after the fluidising bed reactor and before thefeeding in of fresh methane, that a certain amount of gas is transferredoutward, so that the hydrogen partial pressure in the circulation gasremains constant. The amount of the gas to be transferred outward isprimarily oriented to the decomposition of the methane in the depositionor infiltration reactor. The gas transferred outward can be used as highcalorie fuel gas and under certain circumstances also for heating thedeposition reactor.

Nevertheless, a certain percentage of hydrogen remains in thecirculation gas. Hydrogen is not designated as an inert gas since itaffects both decomposition reactions of the methane in the gas phase andalso deposition reactions of the carbon on the substrate surface. In thechemical gas phase infiltration a certain hydrogen partial pressure iseven advantageous. It reduces the decomposition of methane in the gasphase and increases the diffusion in the pores because of its small molmass. With the aid of examples it will be shown that an even better porefilling can be achieved with hydrogen in the circulation gas thanwithout hydrogen.

The foregoing considerations have been concentrating on the chemical gasphase infiltration of carbon. They are equally valid for the chemicalgas phase infiltration of other refractory substances, especially forthe chemical gas phase infiltration of silicon carbide. Also in thiscase, higher and in part cyclical compounds can be formed, resemblingbenzene which must be removed from the circulation gas and which can inthe same way be removed in a fluidising bed reactor. In the depositionof silicon carbide, the reaction gas which arises is not hydrogen buthydrogen chloride. This must also be removed in the same way thathydrogen is removed in part in the carbon deposition from the process orcirculation gas.

Through the formation of hydrogen chloride and its circulation system isdivergent from equation (2) in that it is also possible to usemethyltrichlorosilane as the educt gas, as this reacts with hydrogenchloride with a high level of selectivity to SiCl₄ and CH₄ (equation(10)):

CH₃SlCl₃+HCl→SiCl₄+CH₄   (10)

The advantage of methyltrichlorosilane is that it is easier to handle.

According to the invention it is possible that the infiltration of theporous structure is interrupted when carrying out a mechanical cleaningof the surface of the porous structure. However, according to a preferedembodiment of the invention the process of infiltration is carried outwithout interruption when carrying out a mechanical cleaning of thesurface of the porous structure.

A diagram for a complete system for chemical gas phase infiltrationaccording to the invention is represented in FIG. 3.

The advantages of the process according to the invention could beconfirmed by experimental evaluations. Examples of experiments on theinfiltration of C confirming effects which are taken advantage of by theprocess according to the invention are shown below. Reference is made tothe drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawing there is shown:

FIGS. 1A to 1C a schematic diagram of an infiltration reactor for partswith large diameter or length to thickness ratio;

FIG. 2 a horizontal section through the fitted part of the infiltrationreactor for cylindrical parts with medium to small diameter/length tothickness ratio;

FIG. 3 a schematic flow chart in the circulation system with depositionof intermediate products;

FIG. 4 a section through the test reactor, schematic representation;

FIG. 5 the cumulative pore volume of the porous, cylindrical substratein dependency on pore diameter;

FIG. 6 the relative mass increase of the porous, cylindrical substratein dependency on the infiltration time under the following conditions:p_(total)=20 kPa, T=1100° C., T=0.16 sec., methane argon mixtures ofvarious composition;

FIG. 7 the initial rate of the relative mass increase in dependency onthe methane partial pressure under the method conditions correspondingto FIG. 6;

FIG. 8 a graph of the results according to FIG. 7 and equation 12;

FIG. 9 the degree of pore filling in dependency on methane partialpressure under the method conditions corresponding to FIG. 6, wherebythe pore filling levels are calculated according to equations (13) and(14);

FIG. 10 the degree of pore filling in dependency on the infiltrationspeed, represented with the aid of the initial rate of the infiltration;

FIG. 11a the cumulative pore volume of a porous, cylindrical substrateafter 76 hours infiltration under the following infiltration conditions:p_(total)=20 kPa, T=1100° C., τ=0.16 sec, P_(CH4)=2.5 kPa, P_(Ar)=17.5kPa;

FIG. 11b the cumulative pore volume of a porous, cylindrical substrateafter 79 hours infiltration under the infiltration conditionsp_(total)=20 kPa, T=1100° C., τ=0.16 sec, P_(CH4)=10 Pa, P_(Ar)=10 kPa;

FIG. 12 the relative mass increase of a porous cylindrical substrate independency on the infiltration time under the infiltration conditionsT=1100° C., p_(total)=PCH4, τ=0.16 sec;

FIG. 13 the relative mass increase or a porous cylindrical substrate independency on the infiltration time under the infiltration conditionsT=1100° C., p_(total)=20 kPa, P_(H2)=2.5 kPa, P_(CH4) ascending, P_(Ar)descending;

FIG. 14 the initial speed of the relative mass increase in dependency onmethane partial pressure under the infiltration conditions correspondingto FIG. 13;

FIG. 15 the degree of pore filling in dependency on the methane partialpressure under the infiltration conditions corresponding to FIG. 13;

FIG. 16 the degree of pore filling in dependency on the initial rate ofinfiltration under the infiltration conditions corresponding to FIG. 13;

FIG. 17 the cumulative pore volume of a porous, cylindrical substrateafter 77 hours of infiltration under the infiltration conditionsp_(total)=20 kPa, T=1100° C., τ=0.16 sec. P_(CH4)=15 kPa, P_(CH4)=2.5kPa, P_(H2)=2.5 kPa;

FIG. 18 the relative mass increase of a porous cylindrical substrate independency on the infiltration time under the infiltration conditionsp_(tota)=50 kPa, T=1100° C., P_(CH4)=43.75 kPa, P_(H2)=6.25 kPa;

FIG. 19 the initial speed of the relative mass increase in dependency onmethane partial pressure which in the case of pure methane i.e. the ofthe above curve, corresponds to the total pressure.

DETAILED DESCRIPTION OF THE INVENTION Experimental Example 1

The investigations on the method subject of the invention were carriedout in a vertical short pipe reactor with conical inflow jet. Thisapproximates a technical deposition reactor in accordance with FIG. 2.FIG. 4 shows a schematic sketch of the reactor. The cylindrical, poroussubstrate of aluminium oxide ceramic has a diameter of 16 mm. Theaperture between the substrate and the reactor wall has a thickness of 2mm. The porous substrate is positioned on a cone of solid ceramicstanding in inverse position. At the top it is covered by a plate ofsolid ceramic in order to ensure an infiltration prefered in radialdirection. The conical inflow jets make it possible for the startinggases to enter the cylindrical deposition part of the reactorpractically in non decomposed state. A defined, vortex-free flow of thegas with a short direct contact time at medium gas flow is made possibleby the vertical arrangement, the cone and the concentrated arrangementof the cylinder with thin slots between the wall, the cone and thecylindrical sample; these conditions fulfil the criteria of the methodaccording to the invention.

The porous, cylindrical substrate used in all the methods has a diameterof 1.6 cm, height of 2 cm and a body density of 2.86 g/cm; it iscomposed of an aluminium ceramic. The open porosity is 23.24%; closedporosity not applicable. FIG. 5 shows the pore diameter spread,determined with the aid of mercury porosity measuring. This porediameter spread serves in the following examples as a reference forinfiltrated samples. The infiltration in this and all the followingexamples always takes place at 1100° C. and with a medium direct contacttime of 0.16 s. referring to the cylindrical reactor part.

The first example treats the influence of methane partial pressure at atotal pressure of 20 kPa. The methane partial pressure is varied by theadditional mixing of argon as inert gas. The influence of the inert gasis therefore also represented simultaneously.

FIG. 6 shows the relative mass increase of the sample as a result of theinfiltrated carbon in dependency on the duration of infiltration. Arelative mass increase of 0.15 corresponds to a pore filling of 94% (seelater). FIG. 6 shows that the speed of the infiltration increases withrising methane partial pressure and this in the whole area of thepartial pressure examined of 2.5 to 20 kPa, whereby the latter partialpressure corresponds to the total pressure. This is particularly clearlyrecognisable in the initial rising gradient of the curves[d(Δm/m_(o))/dt]_(t=0).

FIG. 7 illustrates these speeds in dependency on methane partialpressure. The curve follows a clear hyperbolic course i.e. the reactionspeed in dependency on methane partial pressure only follows a law ofthe first order in the case of very small partial pressure, at highpartial pressure the order goes formally towards nil. In this case,saturation absorption has been reached i.e. if the methane partialpressure continues to rise, the deposition speed does not increase anyfurther.

The hyperbolic curve in FIG. 7 can be described in simplified form bythe following equation (11):

[d(Δm/m _(o))/dt] _(t=0)=(k ₁ *p _(CH4))/(k ₂ +k ₃ *p _(CH4))   (11)

It can be seen that for small methane partial pressures, the rate of thereaction is formally of first order, at high methane partial pressureformally of zeroth order, namely then, when saturation absorption hasoccurred.

The inverse of equation 11 is equal to equation 12

[d(Δm/m _(o))/dt] ⁻¹ _(t=0)=(k ₃ /k ₁)+(k ₂ /k ₁)*p _(CH4) ⁻¹   (12)

A plot of the data points in FIG. 7 according to equation 12 yields astraight line (FIG. 8). This confirms the hyperbolic course and therebyalso the saturation adsorption described repeatedly. FIG. 6 shows longerinfiltration times but also that a lower partial pressure of methaneeven after infinitely long reaction times the same relative maximumincrease in mass is obviously not reached as is the case at higherpartial pressures of methane. It follows from the at higher partialpressures of methane recognizable limiting value that the deposition onthe outer surface of the substrate with respect to the relative massincrease is negligible. The curves in FIG. 6 going through the datapoints were drawn by adjustment with the help of the empirical functionin equation 13.

Δm/m _(o)=(Δm/m _(o))_(t=infinity)*(1−exp(−k*t))   (13)

(Δm/m_(o))_(t=infinity)=maximum relative mass increase in the respectiveinfiltration.

Due to the total porosity of 23.24% and the density of the infiltratedcarbon of 2.07 g/cm, the maximum possible mass increase at completeinfiltration of the pores is [(Δm/m_(o))_(t=infinity)]max=0.1594. Theratio of the maximum relative mass increase (Δm/m_(o))_(t=infinity) to[(Δm/m_(o))_(t=infinity)]max yields the degree of pore filling, equation14.

(Δm/m _(o))_(t=infinity)/[(Δm/m _(o))_(t=infinity)]_(max) =PFG   (14)

FIG. 9 shows the degrees of pore filling due to infiltration of thepores as dependent on the partial pressure of methane. It is very clearthat the degree of pore filling increases with the partial pressure ofmethane. This confirms that under the conditions according to theinvention high partial pressures of methane accelerate not only theinfiltration but also lead to an increasingly higher maximum degree ofpore filling.

To round out the picture, one can also represent the degrees of porefilling as dependent on the rate of infiltration; this representation isshown in FIG. 10. This figure shows directly and unambiguously that thehigh rates of infiltration realized according to the method of theinvention make a maximal pore filling possible. And also not only slowrates of infiltration as postulated in the literature, for example W. V.Kotlensky, in Chemistry and Physics of Carbon, Vol. 9, edited by P. L.Walker, P. A. Thrower, Marcel Dekker, New York, 1793, pg. 187:“Theoretically, the most ideal condition for pyrolytic carbon depositionin the pores will be given at infinitely small deposition rate.”Furthermore, the results document very impressively that the use of aninert or a dilution gas for maximum pore filling is as harmful as asmall rate of infiltration.

FIGS. 11a and 11 b show distributions of pore diameters of the porousand cylindrical substrate after infiltration with partial pressures ofmethane of 2.5 and 10 kPa. In the latter case the infiltration wasexamined close to the maximum relative mass increase. The remainingvolume of the pores is 10% (FIG. 11b). A direct comparison with FIG. 11ais not possible, since the infiltration was interrupted at 2.5 kPabefore the maximum relative mass increase was reached. However theeffect of the higher partial pressure of methane is impressive since theinfiltration times were about the same. The micropore volume, which uponfurther infiltration cannot be further infiltrated, at pore diameterssmaller than 0.1 μm generated through infiltration is howeverastonishing. It amounts in the case of small partial pressures ofmethane of 2.5 kPa to 0.095 cm³/g, in the case of higher partialpressures of methane of 10 kPa only 0.063 cm³/g, although in the lattercase the pore filling is almost completed. From this it can be seen thatat smaller partial pressures of methane the pores are closed earlierthan at higher partial pressures of methane. This makes the advantagesof the use of higher partial pressures of methane in the scope of theteaching of the invention very clear. Furthermore the distribution ofpore diameters of the cylinder infiltrated with a partial pressure ofmethane of 15 kPa shows an even smaller micropore volume of 0.59 cm³/g.

Experimental Example 2

This example described the results of experiments with pure methane atincreasing total pressure. The infiltration conditions were otherwisethe same as described in example 1. In other words, the temperature was1,100° C., the persistence time of the gas was 0.16 sec.

FIG. 12 shows the relative mass increase as dependent on the duration ofinfiltration for partial pressures of methane of 20, 30, 50 and 100 kPa.The curves were plotted with the help of equation 13. In view of theclear nature of the results it was elected to forego the maximalinfiltration. They document that an even faster infiltration is achievedwith increasing partial pressures of methane, and that under theseconditions a satisfactory and approximately maximum pore filling can beachieved as well.

In the experiment with 100 kPa methane, soot formation was observedafter 7 hours of infiltration time. This result is not surprising sinceat this point the porosity is largely degraded and thus the surface hasbecome too small for deposition. In order to exploit the high rate ofinfiltration at 100 kPa methane, the pressure must be reduced withprogressive infiltration or the temperature has to decreased. In anyevent the results show that even at 50 kPa partial pressure of methaneunder the above conditions a maximum pore filling can be achieved inless than 40 hours. At 20 kPa partial pressure of methane, 60 hourswould be necessary to achieve the same result.

Experimental Example 3

In example 3 an educt gas mixture containing hydrogen is used accordingto a prefered embodiment of the invention. Hereby, a recirculation ofgas is realized. The following results demonstrate the very positiveeffect of the hydrogen with respect to the maximum pore filling.

The experiments were conducted at a total pressure of 20 kPa usingmethane/argon/hydrogen mixtures at a constant partial pressure ofhydrogen of 2.5 kPa. The reaction temperature was 1,100° C., thepersistence time 0.16 sec.

FIG. 13 shows the relative mass increase of the porous cylindricalsubstrate as dependent on the infiltration time. It is obvious that theinfiltration rate increases with increasing partial pressure of methane.This means that the optimal result is achieved at a partial pressure ofmethane of 17.5 kPa and a partial pressure of hydrogen of 2.5 kPa, thatis in the absence of the inert gas argon.

FIG. 14 shows the initial rate of infiltration [d(Δm/m_(o))/dt]_(t=0) asdependent on the partial pressure of methane. The result is similar tothat in FIG. 7. At the highest partial pressure of methane, saturationadsorption is nearly reached; the infiltration rate is formally ofzeroth order.

Additionally, FIGS. 15 and 16 show the maximum possible relative massincrease as dependent on the partial pressure of methane (FIG. 15) aswell as on initial infiltration rate (FIG. 16). The dependencies differfrom those of the results without hydrogen (FIGS. 9 and 10). However,they document clearly the importance of higher partial pressures ofmethane as taught according to the invention and higher rates ofinfiltration disprove hereby the general view in the state of the artand in the literature (see above). The favorable effect of the additionof hydrogen is confirmed by the results of the distribution of porediameters. The cylinder which was infiltrated with a mixture ofmethane/argon/hydrogen 15/2.5/2.5 kPa was examined (FIG. 17). Itseffective residual porosity is only 9.6%, the micropore volume with porediameters smaller than 0.1 μm only 0.046 cm³/g. This result is achievedespecially through the addition of hydrogen as taught according to theinvention.

The deposition of carbon and, with it, the rate of infiltration isdecreased by the hydrogen, which is a phenomenon known from theliterature. To compensate for this decrease methane/hydrogen mixtures ofhigher total pressure are used according to the invention. The result ofthe embodiment of the process with a total pressure of 50 kPa with amethane/hydrogen mixture of the composition 43.75/6.25 kPa is shown inFIG. 18. The curve trajectory was plotted with equation 13. At thistotal pressure the infiltration is terminated as early as after 40hours. With the same ratio of methane/hydrogen of 7 to 1 but at a totalpressure of 20 kPa, 80 hours—twice as much time—is necessary (FIG. 18).This result emphasizes once more the superior advantage of highpressures and refutes the view as held in the state of the art as wellas in the literature.

Experimental Example 4

After the advantageous effect of the addition of hydrogen on the degreeof pore filling during the chemical vapor infiltration of carbon asshown in example 3, the positive effect of the addition of hydrogen toachieve the conditions of saturation adsorption taught according to theinvention which are essential to the process was confirmed in example 4.The results, analogous to example 2, which were obtained with puremethane at increasing total pressure are compared to results obtainedwith a mixture of methane and hydrogen in a molar ratio of 6 to 1 atincreasing total pressure and otherwise identical conditions.

To document the saturation adsorption, the initial rates of the relativemass increase are reused. These are shown in FIG. 19. In the case of theuse of pure methane, the region of saturation adsorption at a pressureof about 50 kPa is achieved, which could be too high depending on thethickness of the parts or the porous structures to be infiltrated.However, if a methane/hydrogen mixture with a hydrogen content of lessthan 15% is used, the region corresponding to the saturation adsorptionis already reached at a partial pressure of methane of 25 to 30 kPa.

Prefered embodiments of the invention are described for the case inconnection with the drawing.

Embodiment 1

Processes for the chemical vapor infiltration of refractory substancessuch as C or SiC are mainly used in the production of fiber-reinforcedcomposite materials which in the English literature are termed ceramicmatrix composites (CMC). A prefered embodiment of the invention for theproduction of a carbon-fiber-reinforced carbon by chemical vaporinfiltration of carbon in a carbon fiber structure is described:

Felt is used as the carbon fiber structure. The structure has a diameterof 36.5 mm and a thickness of 20 mm, corresponding to a volume of about19 cm³. The initial weight is 3.8 g. In assuming a density about 1.8g/cm³ for the carbon fibers, the fibers have a volume of about 2 cm³.The free pore volume prior to infiltration is thereby about 17 cm³.

The infiltration is carried out as follows:

Total pressure P_(total)=20 kPa, temperature T=1,100° C., persistencetime of the gas in the reaction zone τ=0.33 sec. The gas used is amixture of methane and hydrogen in a molar ratio of 7 to 1. Theconditions are adjusted such that as complete an infiltration aspossible is achieved in an acceptable amount of time. Under theseconditions about 10% of the carbon which is added with the educt gasmethane is deposited in the porous structure. The integration of thefiber structure in the reactor is achieved with the help of a specialmounting of two cm thickness according to FIG. 1. Between the specialmounting and the side retaining borders is an aperture of 2 mm width.

After 6 days of continuous infiltration, the infiltrated fiber structurehas a weight of 36.1 g. Taking into account the density of the depositedcarbon of 2.07 g/cm³, a degree of pore filling of over 92%, or aremaining porosity of less than 8% was found. The medium density is 1.9g/cm³. Under no circumstances can similar results be achieved withprocedures in the state of the art, even after week- or month-longinfiltration. To this come the added difficulty that the infiltrationprocess in the state of the art must be interrupted several times inorder to mechanically clean the surfaces.

Embodiment 2

An infiltration of carbon with technically pure methane is carried out.The total pressure is 20 kPa, the temperature 1,100° C., the persistencetime τ is adjusted to 0.16 sec. The porous structure is subjected to agas flow applied through apertures of 2 mm width. Widths of aperturessmaller than 50 mm yield usable pore fillings under high pressures inthe region of saturation adsorption as taught according to theinvention. By using aperture widths of less than 25 mm, pore fillings inthe region of saturation adsorption are achieved, which are better thanthe ones attainable through common processes, with the high pressurestaught according to the invention. Best results are achieved with regardto pore filling and production speed in a region of 1 to 5 mm, as seenin the present embodiments. The widths of the apertures are chosen to belarger than 1 mm in order to facilitate isobaric pressure conditionswith short persistence times. Insofar as isobaric pressure conditionscan be achieved with narrower aperture widths, these can be smaller than1 mm.

Embodiment 3

The following infiltration conditions are maintained:

Temperature T=1,100° C.

Total pressure P_(total)=26 kPa to 100 kPa

Gas flow with pure methane

Persistence time τ=0.16 sec

As can be seen in FIG. 12, the maximum pore filling can be achieved inthe region of the pressures as defined in this embodiment after 50hours, which corresponds to an acceptable production speed.

Embodiment 4

The following infiltration conditions are maintained for theinfiltration of carbon

Temperature T=1,100° C.

Total pressure P_(total)=26 kPa to 50 kPa

Persistence time τ=0.16 sec to 0.33 sec

Partial Pressure P_(CH4)=⅞ P_(total)

Partial Pressure P_(H2)=⅛ P_(total)

As can be seen in FIG. 18, the pore filling degree, which is close tothe maximum reachable pore filling, under these reaction conditions isalready reached after 30 hours of infiltration.

Embodiment 5

An infiltration of carbon is carried out in connection with theinfiltration conditions used in FIGS. 6 and 9. This means that amethane/argon mixture is used. Partial pressures of methane are usedcorresponding to the regions shown in the curve in FIG. 9, in which gooddegrees of pore fillings can be achieved, in other words above 10 kPa.Deposition rates are maintained at 10% to 25% by setting the persistencetime.

Embodiment 6

The same infiltration conditions as shown in embodiment 5 are used,however the partial pressure of methane is in the region for which thecurve of relative mass increase shown in FIG. 14 flattens out, in otherwords above 15 kPa.

Embodiment 7

For the infiltration of carbon a mixture of methane and hydrogen in aratio of 6 to 1 is used. The temperature 1,080° C. Depending on thepressure, the persistence time is between 0.01 and 0.9 sec. The partialpressure of methane is adjusted in the region for which the curve inFIG. 9 flattens, indicating the presence of conditions in the region ofsaturation adsorption. Therefore a pressure of greater than 15 kPa andsmaller than 30 kPa or, during the process for maintaining especiallyfast production rates, of 25 kPa to 50 kPa is maintained.

Embodiment 8

The same infiltration conditions as described in the above embodimentsare used with the exception that methane is replaced bymethyltrichlorosilane and hydrogen is replaced by hydrogen chloride, andthat the process temperature is adjusted to about 1,000° C.

We claim:
 1. An isobaric and isothermic method of chemical vaporinfiltration of refractory materials into a porous substrate in areaction zone comprising: disposing the porous substrate into thereaction zone; providing a linear flow of an educt gas comprisingdeposable material in the reaction zone, at a reaction temperature and areaction pressure that produces saturation adsorption of the deposablematerial onto the substrate; and wherein the linear flow is adjusted tohave a flow rate such that no more than 50% of the deposable material isdeposited into the porous substrate.
 2. The method of claim 1 whereinthe linear flow is directed from the bottom to the top of the reactionzone.
 3. The method of claim 2 wherein the linear flow of educt gas isfree of admixed inert gas.
 4. The method of claim 2 wherein the poroussubstrate is subdivided in spaced substrate parts, the substrate partsbeing adjusted apart from one another in such a way as to yield widthsof equal spacing therebetween.
 5. The method of claim 1 wherein therefractory material comprises carbon.
 6. The method of claim 1 whereinthe refractory material is carbon.
 7. The method of claim 1 wherein therefractory material comprises silicon.
 8. The method of claim 1 whereinthe refractory material is silicon carbide.
 9. The method of claim 1wherein the educt gas comprises methane or natural gas.
 10. The methodof claim 1 wherein the educt gas is methane or natural gas.
 11. Themethod of claim 10 wherein the educt gas further comprises hydrogen gas.12. The method of claim 11 wherein the ratio by volume of methane ornatural gas to hydrogen gas is between 20:1 to 2:1.
 13. The method ofclaim 11 wherein the ratio by volume of methane or natural gas tohydrogen gas is between 10:1 to 5:1.
 14. The method according to any ofclaims 6, 10, or 11, wherein the temperature in the reaction zone isbetween 1000° C. and 1200° C.
 15. The method of claim 1 wherein theeduct gas comprises a silicon containing compound.
 16. The method ofclaim 15 wherein the silicon containing compound ismethyltrichlorosilane.
 17. The method of claim 16 wherein the educt gasfurther comprises hydrogen gas.
 18. The method of claim 17 wherein themolar ratio of methyltrichlorosilane to hydrogen gas is between 1:1 and1:100.
 19. The method of claim 17 wherein the molar ratio ofmethyltrichlorosilane to hydrogen gas is between 1:2 and 1:10.
 20. Themethod of claim 1 wherein the educt gas is methyltrichlorosilane. 21.The method according to any of claims 8, 20, or 17 wherein thetemperature in the reaction zone is between 900° C. and 1100° C.
 22. Themethod according to any of claims 8, 20, or 17 wherein the educt gasfurther comprises hydrogen chloride.
 23. The method of claim 1 whereinthe educt gas is recirculated.
 24. The method of claim 1 wherein thelinear flow is adjusted to have a flow rate such that no more than 10%to 25% of the disposable material is deposited into the poroussubstrate.